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Efficiency of electrical heat vs. mechanical heating

  1. Jan 10, 2013 #1

    Low-Q

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    I have a question about efficiency of electrical heating vs. mechanical heating.

    Let me explain an example:

    I have two containers with 1 litre of water each.
    In one container I use a electric heater (electrical resistance) 1W.
    In the other container I use an electric motor attached to a whisk where the motor use 1W to run the whisk.

    Except for the efficiency of the electric motor: Which scenario will heat up the water to a given temperature first, given that the water in both initially is the same?

    I ask because I assume that a electric heater (resistance) will radiate wavelength that is not absorbed by the water as well, while the mechanical work will cause friction in the water itself to be heated, and assume this is more efficient.

    Any idea, or facts about this?

    Vidar
     
  2. jcsd
  3. Jan 10, 2013 #2
    the vast majority of the mechanical work will be taken up by bulk motion of the fluid, not heat.
     
  4. Jan 10, 2013 #3

    Low-Q

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    I understand that, but that motion is not loading the motor except for the period the fluid accelerates into that motion. This motion is kinetic energy that is conserved after acceleration, and do not require energy to sustain. However, the loss (friction) that wants to slow down the motion in the water require 1J/s. That is loss which will generate heat.
    If this wasn't the case we put in energy that is not conserved (lost or destroied), and that is not possible. 1W from the whisk is required to fight friction - not to sustain motion.

    Vidar
     
  5. Jan 10, 2013 #4

    russ_watters

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    This isn't true: Low-Q is correct.

    Once the fluid stops accelerating as it is being stirred -- which takes only a few seconds -- all of the energy goes toward combating friction and all goes toward generating heat.

    The electric resistance heater (an immersion heater, right?) heats the water just fine because as an immersion heater, it heats due to conduction. So...

    The immersion heater gets a little head start (a few seconds), then heats the water at the same rate as the stirrer, but the stirrer heats the water more evenly.
     
  6. Jan 10, 2013 #5

    Low-Q

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    Yes, immersion heater is the right description. Thanks for the answer :-)

    Vidar
     
  7. Jan 10, 2013 #6
    well..u have already come to the conclusion..but i want to tell u from another perspective..
    the energy conservation..both electrical appliances have same power requirement.if u study electrical heater..what ever the electrical energy is coming in is going out in the form of heat..from a resistance which is totaly surrounded by water(in conventional heaters) so loses are minimum..but in case of electrical motor..the loses are high..because there is alot of friction in motors..in bearings,etc..due to which energy is lost ..so water will be heated more quickly by electrical heater.. thank you
     
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